Can I use the A2+ USB input with tablets or smartphones like Samsung Galaxy or Microsoft Surface?

Yes. If your tablet or smartphone has a USB output and supports USB audio (refer to the manufacturers’ documentation), then it will work with A2+'s USB input. For smartphones you'll also need an OTG adapter cable to connect from your tablet to the USB input on the A2+. These are generally available from the manufacturer of your tablet/smartphone, local electronic stores, or even online retailers.